This disclosure relates to air separation systems for aircraft, and more specifically to an oxygen sensor for a nitrogen generation system.
Aircraft fuel tanks and containers can contain potentially combustible combinations of oxygen, fuel vapors, and ignition sources. In order to prevent combustion, the ullage of fuel tanks and containers is filled with air with high nitrogen concentration, or nitrogen-enriched air (NEA). A nitrogen generation system (NGS) is commonly used to produce NEA for inerting fuel tanks and containers. An air separation module (ASM) in the NGS separates ambient air into NEA, which is directed to fuel tanks and containers, and oxygen-enriched air (OEA), which is rejected overboard. The ASM typically includes a polymeric membrane for separating ambient air into NEA and OEA.
For a given system, the amount of oxygen in the NEA depends on various parameters such as feed flow and operating pressure. Therefore, an oxygen sensor can be used to ensure that the oxygen concentration in the NEA remains below a prescribed level in order to reduce risk of explosion in fuel tanks and containers. Typically the oxygen sensor is fed by a small slip gas flow from the NEA flowing out of the ASM to the fuel tanks and containers. Once the slip gas flow passes through the oxygen sensor so that the oxygen sensor measures the oxygen concentration in the NEA, the slip gas flow is rejected overboard. Positioning an oxygen sensor in a slip gas flow stream therefore results in a loss of NEA, which could otherwise be used for fuel tank and container inerting. Additionally, the oxygen sensor and slip gas flow take up additional room, which results in the NGS taking up additional space within the aircraft.